1. Field of the Invention
This invention relates to a single crystal gallium nitride (GaN) substrate for producing blue light emitting diodes (LEDs) and blue light laser diodes (LDs) composed of group 3-5 nitride type semiconductors, to a method of producing a single crystal gallium nitride substrate, and to a method of growing a GaN crystal.
Blue light emitting diodes (LEDs) based upon the group 3-5 nitride type semiconductors (InGaN, GaN) have been manufactured, sold and used on a large scale. Almost all the practical nitride type LEDs are made upon insulating sapphire (α-Al2O3) substrates. Sapphire belongs to trigonal symmetry group (a=b=c, α, β, γ<120, ≠90). GaN films and InGaN films are heteroepitaxially grown on a sapphire three rotationally symmetric plane for producing LEDs. On-sapphire LEDs made upon sapphire substrates have very high dislocation density of 109 to 1010 cm−2. Despite great many dislocations, on-sapphire LEDs do not degenerate and enjoy a long lifetime.
Sapphire has, however, some drawbacks as a substrate. Sapphire lacks natural cleavage. Sapphire is an insulator. Lack of natural cleavage incurs a problem of chip-division. A device-fabricated sapphire wafer is cut and separated into individual device chips by mechanical dicing. The mechanical dicing lowers the yield and enhances the cost.
2. Description of Related Art
The most suitable substrate for nitride type (InGaN) LDs and LEDs should be a GaN single crystal substrate which allows InGaN, GaN, AlGaN films to grow homoepitaxially.
Czochralski method and Bridgman method which change a melt polycrystal material into a melt, cool a part of the melt and make a large single crystal bulk solid at a thermal equilibrium, are unavailable for making a GaN single crystal.
A new method of making a thick GaN film on a foreign material substrate (e.g., sapphire) by vapor phase epitaxial growth method was proposed. It is an extension of a film growth method. However, a sapphire substrate which is chemically stable and physically rigid cannot be eliminated after the GaN film has been grown on the sapphire substrate. Thus, sapphire is not pertinent for the substrate for growing GaN films for the purpose of obtaining a freestanding GaN crystal. Recently trials have been done for eliminating sapphire substrates from grown GaN films by a laser. However, the separation of the sapphire substrates from the GaN films is difficult even by high power lasers.
Instead of the sapphire substrate, another candidate which can be eliminated from grown GaN films would be a GaAs substrate. A (111) plane of GaAs has three-fold rotation symmetry. A C-plane GaN film would be grown in vapor phase along c-axis on the (111) GaAs substrate. However, it is found that thick GaN is not grown upon a GaAs substrate. Perhaps differences of lattice constants and thermal expansions between GaAs and GaN cause the difficulty of growing thick GaN on the GaAs substrate. The lattice misfit and the thermal distortion induce large inner stress which forbids a GaN film from growing to a thick crystal. A breakthrough was required for making a thick GaN crystal in vapor phase.
The inventors of the present invention contrived a GaAs-based epitaxial lateral overgrowth method (ELO) for making low-dislocation GaN crystals by preparing a GaAs substrate, making an ELO mask having many small regularly-populated windows on the GaAs substrate, and growing GaN films by a vapor phase growing method on the ELO-masked GaAs substrate. The inventors had filed a series of patent applications based on the GaAs-based ELO methods for making GaN crystal bulks.    1. Japanese Patent Application No. 9-298300    2. Japanese Patent Application No. 10-9008    3. Japanese Patent Application No. 10-102546    (1, 2 and 3 have been combined into a PCT application of WO 99/23693.)    4. Japanese Patent Application No. 10-171276    5. Japanese Patent Application No. 10-183446
An ELO mask is made by preparing a three-fold rotation symmetric GaAs (111) substrate, piling a thin SiN film (e.g., 100 nm thickness) on the GaAs substrate, and forming many small regularly-distributed striped or dotted windows on the SiN film by etching.
Then, epitaxial GaN films are grown on the non-masked GaAs substrate in vapor phase at a high temperature. The GaN films make normal cones on the windows. The GaN cones overrun the edges of the ELO mask, crawl on the mask and meet with other GaN films on the ELO mask on a bisector.
After two neighboring GaN films meet on the bisector, the growing direction changes. The GaN films grow in the vertical direction along an c-axis. It is a C-plane growth which maintains the C-plane as a unique, smooth, flat surface. The C-plane growth is a well known-method of GaN growth. A long-term vapor phase growth makes a thick GaN/mask/GaAs samples of several hundreds of thickness. Then, the mask and the GaAs substrate are eliminated by, for example, aqua regia.
The epitaxial lateral overgrowth (ELO) method which makes use of a mask having many windows can produce a GaN crystal of 1-2×107 cm−2 dislocation density. Reduction of dislocations is insufficient. ELO-made GaN crystals are unsatisfactory as a GaN substrate upon which InGaN type LDs are fabricated. InGaN-LDs require better GaN crystals of far smaller dislocation density.    6. Japanese Patent Laying Open No. 2001-102307 (Japanese Patent Application No. 11-273882)
Facet growth was proposed in the document 6 by the same inventors as the present invention. All the known GaN growth has been C-plane growth which maintains a smooth, flat C-plane as a surface of c-axis growing GaN. The document 6 denied the conventional C-plane growth and advocated facet growth which makes facets and pits composed of the facets on a growing GaN surface and maintains the facets and pits without burying pits. A GaN facet grows in a direction normal to the facet. Although an average direction of growth is a c-axis direction, microscopic growing directions are non-c-axis directions.
FIG. 1 to FIG. 3 show our previous facet growth. In FIGS. 1(a) and (b), a GaN crystal 2 is growing in a c-axis direction, having a C-plane top surface 7. Crystallographical planes inclining to the C-plane are called facets 6. The facet growth forms facets and maintains the facets without burying facets. In the example of FIG. 1, six facets 6 appear and form a polygonal reverse cone pit 4 on the C-plane surface. The pits built by the facets are hexagonal cones or dodecagonal cones. Hexagonal pits are formed by six-fold rotation symmetric facets of either {11-2 m} or {1-10 m} (m: integer). Dodecagonal pits are composed of {11-2 m} and {1-10 m} (m: integer). Although FIGS. 1(a) and (b) show the hexagonal pit, dodecagonal pits appear prevalently.
To form facet pits, to maintain pits and not to bury pits are the gist of the facet growth. A facet 6 displaces at a direction normal to the facet. A dislocation extends along a growing direction. A dislocation extending along a c-axis and attaining the facet turns an extending direction in a horizontal direction parallel to the facet and reaches a crossing line 8. The crossing lines 8 include many dislocations. As the top surface moves upward, loci of the crossing lines 8 make crossing planes 6 which meet with each other at 60 degrees. Planar defect assemblies 10 are formed on the crossing planes. The planar defect assemblies 10 are a stable state.
Some dislocations attaining to the crossing line turn an extending direction again inward, move inward along the rising slanting crossing line 8 and fall into a manifold point D at a pit bottom. The dislocation substantially runs inward in the horizontal direction. A linear defect assembly 11 is formed along the manifold point D at the bottom of the pit. The linear defect assembly 11 is less stable than the planar defect assemblies 10.
The inventors noticed that the facet growth method has still problems for producing GaN wafers for making LD chips.
The facet growth can gather dislocations into a narrow volume by making facet pits, growing a GaN crystal without burying facets, gathering dislocations into the bottoms of pits. Dislocations do not necessarily converge to a single point but diffuse outward. When a plurality of 100 μmφ pits are formed, dislocations converge to a narrow spot at a bottom of a pit somewhere. But at other regions, dislocations diffuse till about 30 μmφ wide range. The 30 μmφ diffusion makes a hazy dislocation nebula.
This means that once converged dislocations disperse again to a hazy nebula of dislocation. It was confirmed that lines of the hazy nebulae diffusing from the pit bottom assembly include many dislocations.
Hazy dislocation nebulae have very high dislocation density of 107 cm−2 which is ten times as much as an average dislocation density (106 cm−2). Such high dislocation density 107 cm−2 of the hazy dislocation nebulae is insufficient for making use of the GaN crystal as an LD substrate for making LD devices. An LD substrate requires low dislocation density less than 106 cm−2. The occurrence of the hazy dislocation nebulae is the first problem of the previous facet growth.
The second problem is planar defect assemblies which are born by gathering dislocations to the pit bottoms and inclining to each other at 60 degrees. The planar defect assemblies dangle from the crossing lines 8. 60 degrees spacing planar defect assemblies 10 have six-fold rotation symmetry. The planar defect assemblies include high density dislocations. In addition to the hazy dislocation nebulae, the radially extending planar defects assemblies are a serious problem for an LD substrate, since the planar defects would induce degeneration and would restrict lifetime of LDs. An LD substrate requires a reduction of the planar defect assemblies.
The last problem is more fundamental. Occurrence and distribution of pits are stochastic, accidental and unprogrammable. The distribution of pits are entirely at random. The previous facet growth method which reduces dislocations by growing facet pits without burying, has a weak point of undeterminable positions of pits. It is impossible to previously determine or know the spots at which facet pits happen. An accident makes a pit at an undetermined spot. The positions of pits are stochastic variables. The formation of pits are uncontrollable. Uncontrollability of pit positions is a serious problem.
Three matters aforementioned are the problems to be solved by the present invention. In short, the objects of the present invention are converged into three matters;    (1) Reduction of hazy diffusion of dislocations from the defect assemblies of the centers of facet pits (FIG. 3(2)).    (2) Annihilation of planar defects occurring at the centers of the facet pits (FIG. 1(b)).    (3) Controlling of positions of defect assemblies at the centers of facet pits.
All the three are difficult problems. Difficulties are again clarified here. The serious problem of the previous facet growth of the inventors which maintains facets and pits without burying the facets was an unstable state of defect assemblies at pit bottoms. FIGS. 3(1) and (2) show the state of defect assemblies of our previous facet growth method. Accidentally a pit 14 with facets 16 occurs somewhere on a growing GaN film surface. The positions of the pits cannot be determined previously. Occurrence of pits and points of occurrence of pits fully depended on contingency. Occurrence of pits and positions of pits were uncontrollable. In accordance with the GaN growth in an upward direction, facets 16 rise and dislocations move in the horizontal direction to the center of the pit 14. A dislocation bundle 15 is formed at the bottom of the pit 14. As shown in FIG. 3(2), the dislocation bundle is neither encapsulated nor arrested by anything. Ephemerally assembling, individual dislocations in the dislocation bundle have a strong tendency of diffusing and dispersing outward again by mutually acting repulsive force.
The present invention intentionally produces crystal boundaries and makes the best use of the boundaries for manufacturing low dislocation density GaN single crystals. FIG. 4 shows the action of the facets, pits and grain boundary of reducing dislocations. A growing GaN crystal 22 has a pit 24 consisting of facets 26. The facet pit 24 is not buried but maintained during the GaN growth. Top of the crystal is a C-plane surface 27. The facet pit 24 has a central bottom 29. When the GaN film further grows, facets 26 grow in the direction vertical to the facets 26. Dislocations are swept in the centripetal, horizontal directions to the pit center. The directions of dislocations are parallel to the C-plane 27. The dislocations attracted to the center are affiliated to dislocation assembly 25 at the pit bottoms 29. The dislocation assembly 25 is encapsulated by boundaries (K) 30. The dislocation assembly is called a “closed defect accumulating region (H)”, since the region arrests, accumulates and is closed by the boundary (K). The closed defect accumulating regions (H) 25 have a very significant function of attracting, absorbing, annihilating and accumulating dislocations permanently.
Once dislocations are arrested, the dislocations cannot escape from the closed defect accumulating regions (H). Thus, the region (H) is “closed”. The region (H) is closed by the grain boundary (K).
This invention preliminarily forms seeds on an undersubstrate, makes closed defect accumulating regions (H) following the seeds, and proceeds facet growth. The facet growth sweeps dislocations of the other regions and stores the swept dislocations into the closed defect accumulating regions (H). The closed defect accumulating regions (H) hold many dislocations captive. The seed-defined closed defect accumulating regions (H) and the facet growth enable us to accomplish all the aforementioned three purposes,    (1) a decrease of foggy dislocations leaking from the centers of the facet pits,    (2) an elimination of planar defect assemblies at the centers of the facet pits, and    (3) a control of the positions of the defect assemblies at the centers of the facet pits.
However, some problems remain. The closed defect accumulating regions (H) take various crystal structures. Sometimes the closed defect accumulating regions (H) are polycrystal. Polycrystalline closed defect accumulating regions (H) have a tendency of diminishing and extinguishing midway. If the polycrystalline closed defect accumulating regions (H) survive, the polycrystalline closed defect accumulating regions (H) induce microcracks at the boundaries (K). The microcracks are caused by random differences of thermal expansion in the polycrystal, while the other parts are a single crystal. Microcracks break GaN substrates in wafer processes. Thus, the polycrystalline closed defect accumulating region (H) is not best.
A single crystal closed defect accumulating region (H) with slanting axes or slantingly inverse axes also incurs microcracks at the interfaces. The reason why the microcracks occur is also the thermal expansion anisotropy different from the other (0001) single crystal parts. Polycrystalline or slanting oriented single crystal closed defect accumulating regions (H) are not the best ones owing to the microcracks caused by the thermal expansion discrepancy between the (H) regions and the other regions.
A single crystal closed defect accumulating region (H) with exact inverse axes (precise 180 degree rotating, antiparallel to the other parts) incurs no microcracks at the interfaces. It is because the thermal expansion anisotropy in the closed defect accumulating region (H) exactly coincides with anisotropy of the other parts, since the orientations are exactly inverse. Due to no probability of the microcracks, the orientation-inverse single crystal one is the best closed defect accumulating region (H).
However, properties and orientations of closed defect accumulating regions (H) depend upon accidents. It is difficult to always form orientation-inverse, antiparallel single crystal closed defect accumulating regions (H) even on dot-seeded undersubstrates.
For example, in the case of forming an SiO2 seed mask on a sapphire undersubstrate, sometimes polycrystalline closed defect accumulating regions (H) happen on the seed. Other times slanting-orientation single crystal closed defect accumulating regions (H) grow on the seed. Once formed slanting-orientation closed defect accumulating regions (H) often disappear halfway. Sometimes closed defect accumulating regions (H) of orientation-inverse single crystals mixed with polycrystals are born on the dotted seeds.
The conditions of making desired orientation-inverse (antiparallel) closed defect accumulating regions (H) on the dot seed have not been known yet.
The inventors have thoroughly investigated the cases in which orientation-inverse closed defect accumulating regions (H) have been formed on the seeds. The inventors sought for a way how to build the orientation-inverse single crystal closed defect accumulating regions (H) on the dotted seeds with high probability and found the way of forming the orientation-inverse (H) region. Then, the fourth purpose is,    (4) formation of orientation-inverse single crystal closed defect accumulating regions (H) with high probability.